Aspects of the present invention are directed to epitaxial source/drain contacts that are self aligned to gates for carbon nanotube-based field effect transistors (CNTFETs).
Switching devices based on carbon nanotubes (CNTs) have enormous potential due to the high carrier mobility and good short channel effects stemming from the thin body of the CNT. For example, CNTFETs have been proposed as a potential post-silicon complementary-metal-oxide-semiconductor (CMOS) solution for dense logic applications. For this potential to be realized, a method for building CNTFETs at dense pitch is necessary. The high mobility of the ideal CNTFET enables width scaling and good short-channel effects of the ideal CNTFET enables gate length scaling. However, one of the many additional challenges a CNTFET-base technology must overcome is compatibility with the high layout density that traditional silicon CMOS technology currently supports. In particular, for high layout density layouts, the source/drain and gate contacts to the switching device built around each CNT must all be precisely positioned.
Gate pitch scaling requires a manufacturable device structure in which the source/drain is self-aligned to the gate. Such self-alignment eliminates the variability in parasitic resistance and capacitance caused by misalignment of the source/drain to the gate and it also eliminates the area penalty of having to include a margin for misalignment in the layout. In silicon CMOS, this precise positioning is enabled by using gate shadowing to define implanted junction profiles and by the self-aligned silicide process. For CNTFETs, these methods are often inapplicable.
CNTFETs with source-drain contacts that are self-aligned to the gate (SA CNTFET) have been demonstrated using directional evaporation, chemical doping and electrostatic doping. In each process, however, problems remain. For example, the process window for directional evaporation is too narrow for manufacturability, chemical doping of CNT source/drain contacts continues to be an active area of research but results remain irreproducible and inconsistent and electrostatic doping works relatively well but is not ideal because the back gate required for electrostatic doping requires additional layout area and introduces a large parasitic capacitance.